Internal combustion engines may include water injection systems that inject water into a plurality of locations, including an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Injecting water into the engine intake air may increase fuel economy and engine performance, as well as decrease engine emissions. When water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to the water. This heat transfer leads to evaporation, which results in cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx, while a more efficient fuel mixture may reduce carbon monoxide and hydrocarbon emissions.
Water injection may be controlled based on feedback from an exhaust oxygen sensor. In particular, the injected water may generate a dilution effect, and the oxygen sensor may learn a change in the oxygen content of the exhaust gas due to the presence of the added dilution. For example, as shown by Leone et al. in U.S. Pat. No. 8,960,133, an amount of water injected for knock relief is controlled based on excess oxygen being detected by an exhaust oxygen sensor.
However the inventors herein have recognized potential issues with such an approach.
The approach of '133 relies on a single water injector being operated at a time. Consequently, the water injection error learned via the exhaust oxygen sensor is attributed to only that water injection, which is then duly corrected. In engine systems configured with multiple water injectors, at any given time, different amounts of water may be injected into distinct engine locations to address charge cooling, component cooling, and charge dilution (for example, concurrently). This may complicate assigning of water injection errors to distinct injections and performing of a water injection correction. The system may instead need to rely on multiple sensors positioned at distinct locations, adding cost and complexity. As another example, each water injector may have distinct injection limits due to the location of water injection as well as the prevalent engine operating conditions. For example, the output of a manifold water injector may be constrained by the manifold humidity, which in turn is a function of the manifold air temperature and pressure. The difference in injection limits may complicate the water injection error correction. Errors in water injection can result in engine knock, cylinder misfire events, as well as combustion instability.
In one example, some of the above issues may be addressed by a method for an engine comprising: injecting water into distinct engine locations responsive to each of an engine dilution demand and an engine cooling demand; and correcting a total water injection amount based on feedback from an exhaust oxygen sensor operating in a variable voltage mode. In this way, water injection errors may be better addressed.
As an example, water may be concurrently injected into multiple locations of an engine to meet each of an engine cooling demand (e.g., for knock relief) and an engine dilution demand. For example, water may be injected into an intake manifold via a manifold injector, and into an intake port via a port injector. A total amount of water that is commanded to be injected into the engine, as well as a ratio of the total amount of water commanded to each location, may be based on the engine cooling demand relative to the engine dilution demand. Further, the engine dilution demand may be met in coordination with the usage of exhaust gas recirculation (EGR). An exhaust oxygen sensor may be operated in a variable voltage mode before and after the water injection, and the excess oxygen in the exhaust measured by the sensor may be attributed to the total amount of water that was received in the engine. A water injection error may then be determined based on the total commanded amount and the total measured amount. An engine controller may then learn the injection limits for each water injector, including upper and lower thresholds, and error tolerances. Based on the initial water injection ratio, the magnitude and directionality of the water injection error, as well as the injection limits for each water injector, water injection commands to the distinct injectors may be updated. For example, if the error requires an additional amount of water to be added, and the error is not symmetrically divisible between the two injectors due to an injection limit of one injector being reached, the error may be asymmetrically divided. In one example, the error may be addressed by increasing the water injection amount of the manifold injector until a manifold water saturation point is reached, after which the error may be compensated by increasing water injection via the port injector.
In this way, water injection can be used as a surrogate for EGR as well as for knock relief, simultaneously, and an exhaust oxygen sensor can be used for feedback control of the liquid EGR and the knock relief. The technical effect of using feedback from the oxygen sensor to determine a total water injection error, and compensating for the error by distributing the error amount between distinct water injectors based on their individual injector limits, is that a water error may be better compensated for. By using water to meet an engine dilution demand and an engine cooling demand, knock and combustion instability issues may be addressed with reduced reliance on spark retard, improving fuel economy. By operating an exhaust oxygen sensor in a variable voltage mode, excess water in the engine from all the water injection locations may be dissociated and a total change in oxygen amount can be associated with the total water injection. By relying on an existing exhaust oxygen sensor for feedback control of water injection from multiple injectors, the need for dedicated sensors, including sensors for each distinct water injection, is reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.